Implantable electrical stimulation devices have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and implantable cardiac defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal cord stimulation (SOS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes. Deep brain stimulation has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and it has also recently been applied in additional areas such as movement disorders. In recent investigations, peripheral nerve stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes, and a number of additional applications are currently under investigation. Finally, functional electrical stimulation (FES) systems such as the Freehand™ system by NeuroControl™ Corporation of Cleveland, Ohio have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Current implantable electrical stimulation systems typically consist of a system with electrodes on a lead, separate from but connected to an implantable pulse generator (IPG) that contains the power source and the stimulation circuitry. A number of these systems have multiple programmable electrodes, allowing each electrode to be configured as an anode, a cathode, or as an open circuit (i.e., electrically disconnected). However, these types of leaded systems have several disadvantages. The implantation procedure may be rather difficult and time-consuming, as the electrodes and the IPG must usually be implanted in separate areas and the lead must be tunneled through body tissue to connect to the IPG. Also, the leads are typically thin and rather long and are thus prone to mechanical damage over time. Additionally, many conventional systems typically consist of a relatively large IPG, which can have a negative cosmetic appearance if positioned subcutaneously.
Neurons typically propagate signals in one direction. Peripheral nerve fibers that propagate signals away from the central nervous system (CNS, i.e., the brain and the spinal cord) and towards the periphery and viscera are referred to as efferent nerve fibers. Peripheral nerve fibers that propagate signals away from the periphery and viscera and towards the CNS are referred to as afferent nerve fibers.
Efferent impulses may initiate a variety of actions, from movement of a muscle to initiation of changes in the heart rate or force of contraction or in the level of constriction of the vascular smooth muscle in arterioles. Through increasing or decreasing the activity of efferent fibers, the CNS can, for example, alter the blood pressure by changing the characteristics of the cardiovascular system.
Afferent impulses from specialized nerve endings or receptors inform the controlling neurons in the CNS about characteristics of the system, e.g., if a limb is feeling pain or if blood pressure is high or low. Most peripheral nerves contain both afferent and efferent nerve fibers.
A typical individual neuron consists of a soma (i.e., cell body), which contains the nucleus of the cell; dendrites, which receive input from pre-synaptic neurons; and an axon, which send signals via axon terminals (i.e., the distal portion of the axon) to post-synaptic neurons (or to effector cells, e.g., muscle fibers). An action potential is initiated at the initial segment of the axon (i.e., the proximal portion of the axon) when triggered by input from the dendrites. An action potential is an electrochemical signal that propagates from the initial segment down the axon to the axon terminals. Such propagation is referred to as orthodromic. (Orthodromic is defined as “of, relating to, or inducing nerve impulses along an axon in the normal direction.”) Action potential propagation in the opposite direction is referred to as antidromic. (Antidromic is defined as “proceeding or conducting in a direction opposite to the usual one—used especially of a nerve impulse or fiber.”)
In a neuron at rest, i.e., that is not propagating an action potential, the inside of the axon is negatively charged relative to the outside of the neuron, i.e., the membrane of the axon is at a negative resting potential.
When the soma receives sufficient stimulation at its associated dendrites, it initiates an action potential at the initial segment, which travels orthodromically down the axon. An action potential is initiated and propagated by opening channels in the axon membrane to allow positive charge (e.g., sodium ions) to enter the axon. This causes the voltage of the inside of the axon to become positive, i.e., it depolarizes a segment of the axon. Depolarization of one part of the axon causes depolarization of an adjacent patch of axon; this mechanism allows a wave of depolarization to sweep down the axon. After a brief period of depolarization (e.g., approximately 1 msec), the axon membrane automatically repolarizes to return to a resting state.
Electrical stimulation causes depolarization of the local axon membrane and may be used to initiate action potentials. For instance, electrical activation of an axon performed near the middle of an axon (i.e., not at the initial segment) produces two action potentials. One action potential propagates orthodromically, while the other propagates antidromically.